1. Field of Invention
The present invention relates to the field of CO sensors and methods of making CO sensors. Specifically, the present invention relates to novel CO sensors made from hydrated metal oxides, and in particular CO sensors made from hydrated ruthenium oxide, which operate at a temperature range of from about 25° C. to about 300° C.
2. Description of Prior Art
Carbon monoxide (CO) is a toxic gas that is one of the major pollutants associated with automotive emissions, combustion processes, fires, manufacturing of natural gas, and a host of other industrial activities. At concentrations higher than 15 ppm, CO is dangerous to the human body. Consequently, reliable, low-cost, CO sensors having high sensitivity, selectivity, and acceptable response and recovery times at low temperatures, and which require low energy consumption are needed for environmental safety and industrial control applications. One of the most basic CO detectors utilizes a chemical compound that changes color in the presence of CO. Although inexpensive, these colormetric CO sensors have high detection thresholds and are not suited for quantitative measurement of varying CO concentrations. Other CO sensing technologies are based on optical or electrochemical detection of CO. Sensors employing optical detection technologies are usually very expensive. Consequently, technologies relying on electrochemical detection of CO are more widely used in conventional CO sensors.
Many different apparatuses can be used in the electrochemical detection and measurement of CO species in a gaseous mixture. Several of these employ either a wet or dry chemical cell to detect and measure CO concentration. For example, certain devices are analogous to fuel cells and operate on principles associated with ionic conduction or charge transfer. Such devices have a basic electrochemical cell configuration and contain two electrodes and an electrolyte (in either a solid or liquid form). The electrochemical cell oxidizes CO at one electrode and reduces oxygen or some other species at the other electrode. A current or voltage related to the amount of CO present is produced, and this can be measured using amperometric or capacitive techniques. Although many types of electrochemical sensors are relatively cheap, reliable, and highly selective to CO detection, many also have a disadvantage in that they are inherently sensitive to a wide range of substances and are susceptible to producing erroneous responses.
Other apparatuses are solid-state devices, which rely on conductive, semiconductor, or a mixture of these two materials to detect and measure CO species electronically. Producing a CO sensor from semiconductor materials is advantageous because such devices can be easily integrated into the design and manufacture of computer chips at low costs. Furthermore, simple electronics can be used to easily monitor the output of such devices using straightforward techniques. Certain devices utilize a semiconductor material and operate on a sensing principle involving chemisorption of CO on the surface of the semiconductor film or substrate. The adsorption of CO on the surface of the semiconductor changes the material's electrical resistance and this change can be measured and calibrated against the concentration of CO that is present.
Several resistive-type CO sensor devices have been constructed from crystalline metal oxide semiconductor (MOS) materials, including, for example, indium oxide, titanium dioxide, and tin oxide. A limitation associated with these MOS materials is that any CO sensor constructed from these materials requires a heating device to operate. This is because the room temperature resistance of these MOS materials are too high for construction of a room temperature CO sensor to be practical. Consequently, heating is required to promote sufficient oxygen vacancies for these MOS materials to be conductive. For example, CO sensors constructed from titanium dioxide require elevated temperatures of up to 350° C. or higher for optimum operation. Unfortunately, the elevated operating temperatures cause gradual changes to the MOS crystalline structures resulting in long term instability of the sensor devices. Other CO sensors have been constructed from tin oxide, and these have a typical operating temperatures of from 200-250° C. Furthermore, even though the room temperature conductivity of tin oxide can be increased by doping, the doping operation is detrimental in that it decreases the tin oxide's sensitivity to measuring CO. CO sensors with operating temperatures of around 80° C. have been constructed from cobalt oxide. However, it is impractical to use this material as a low-cost room temperature CO sensor because cobalt oxide has an extremely high room temperature resistance. The high room temperature resistance of cobalt oxide necessitates that any CO sensor so constructed would require more sophisticated electronic devices and techniques to monitor the CO sensor's output.
Thus, there is a continuing need for low-cost, easily integratable, CO sensors that operate at ambient temperatures requiring no power source for heating the sensor, to monitor combustion environments to meet government regulations and minimize negative effects of CO on ecosystems and on health.